Thermocouples are commonly used in the aerospace industry to instrument and measure temperatures in critical components of engines, turbines or other high-performance machines. A thermocouple consists of two dissimilar metal conductors that are electrically bonded to form a junction at the point where temperature measurement is to take place; when the temperature at the junction differs from the temperature at a different reference location in the circuit, a voltage having a known relationship to the temperature and the thermocouple metals used develops across the two conductors. Thermocouples are very simple and reliable, but ordinary electrical insulation on their conductors would quickly be destroyed by the high temperatures encountered in the hot sections of some engines or turbines. Thermocouples for such applications therefore are manufactured using an outer sheath or tube made of an alloy that has a very high melting temperature and is hollow, within which the conductors are insulated with nonconductive magnesium oxide powder which does not melt at the temperatures involved. The thermocouple junction is located just inside the tip of the sheath, which commonly is closed at the end to protect it. This configuration is referred to herein as a “hard line” thermocouple; some can tolerate temperatures of several thousand degrees.
Developmental testing of new engine or high-performance machinery designs may require instrumentation with dozens or hundreds of thermocouples located in carefully chosen locations. The hard lines are stiff and are formed, routed and mounted in place along selected paths on the engine or machine under test. The data collection equipment to which the thermocouples connect is located external to the enclosure or cell in which the engine or machine is operated, but the hard lines will not tolerate repeated flexing, so after exiting the hot section(s) and reaching locations where temperatures are sufficiently benign to allow use of more conventional electrical insulation, the hard line thermocouple conductors are transitioned to flexible extension leads which then egress to the data collection equipment located near the engine or machine test cell. The current method of making this transition typically involves manual crimping of a strain relief device between the flexible extension leads and the hard line sheath, soldering of the extension lead wires to the wires exiting the hard line sheath, insulating the soldered connections using epoxy, and manual crimping of an additional outer protective sleeve over the inner strain relief device and insulated connections. Due to the custom nature of the testing that requires such instrumentation, the manual crimping process typically is performed on site in the engine or machine test cell. The wires exiting the hard line sheath are very fine gauge (as small as 0.006 inch diameter), and the soldering, insulating, epoxy cure times, and manual crimping processes add up to a difficult and time-consuming process that requires skill, careful technique and attention to delicate details. Despite careful technique, shorting of spliced leads or breakage of a fine lead where it exits the hard line sheath still sometimes occurs, rendering that particular thermocouple useless. If temperature measurements from that particular thermocouple are critical, then (depending on its location) replacement of the thermocouple may require either partial or total disassembly of the engine or machine—an expensive loss in terms of both time and money. Even if no wire breakage or shorting occurs, the process of carefully soldering, insulating and crimping the many leads and sleeves is tedious, non-ergonomic, and causes manual fatigue. If the extension leads must be changed, the leads must be cut and the new leads rejoined, reinsulated and re-crimped, with attendant increased risk of a short or breakage.
What is needed is a device that provides a means of rapidly, easily and reliably transitioning hard line thermocouple conductors to flexible extension leads without requiring manual soldering, insulating and crimping.